Most metal parts operate in an environment which eventually leads to corrosion or the creation of stress induced cracks, thereby reducing the useful life of such parts. It is known that peening the surface of metal parts can induce compressive residual surface stresses, thereby increasing the resistance of the part to fatigue, cracking and corrosion. Numerous methods exist which relate to peening the exterior surface of metal parts. These methods, however, are not applicable to peening the internal surface of hollow parts because such methods fail to take into account the peculiar difficulties associated with peening the internal surface.
U.S. Pat. No. 2,460,657 addressed some of the distinctive characteristics associated with peening the internal surface of a hollow part. Specifically, that patent taught that vibrating the hollow part produces repeated impact between the peening elements and the internal surface of the hollow part. Additionally, U.S. Pat. No. 2,460,657 suggested that the peening elements' vibratory motion is largely determined by their own natural frequency, but that patent does not indicate at which frequency the hollow part must vibrate in order to induce the desired residual stresses on the internal surface of a hollow part. In order to induce compressive residual stresses, the peening elements must contact the internal surface at certain velocities. The prior art, however, fails to teach one how to determine the vibration frequency and acceleration at which the hollow part must vibrate in order to cause the peening elements to contact the internal surface at such desired velocities. Specifically, the devices used to vibrate parts, such as shaker tables, typically have two controllers, namely a frequency controller and an acceleration controller to control its vibrational movement. The frequency controller sets the shaker table's vibration frequency (.omega.), and the acceleration controller sets the maximum sinusoidal acceleration (a). It should be understood that if the vibration frequency is known, then the acceleration can be replaced by vibration amplitude (A) because acceleration is equal to the product of the vibration amplitude and the square of the frequency (i.e., a =.omega..sup.2 A). Hence, acceleration and vibration amplitude are interchangeable, but for the purposes of this invention, the inventor shall consistently refer to acceleration rather than amplitude because the devices used to vibrate parts typically refer to acceleration rather than amplitude. It should also be understood, that as the hollow part vibrates, its instantaneous acceleration changes, but the maximum acceleration remains constant, which is hereinafter referred to as the "constant sinusoidal acceleration."
Furthermore, U.S. Pat. No. 2,460,657 indicated that the frequency of the impact between the peening elements and the hollow part should be out of step with the vibration frequency at which to vibrate the hollow part. That patent, however, did not teach how to determine or calculate the acceleration at which to vibrate the hollow part in order to produce a maximum impact rate between the peening elements and the hollow part wherein the impact rate is the rate of impact between the peening element(s) and the hollow part. Moreover, U.S. Pat. No. 2,460,657 indicated that the impact rate is determined by the peening elements own natural frequency of vibration, which is a function of the relative proportions of the peening element(s) and the hollow part, as well as their material, thereby suggesting that one could alter the proportion and material of the peening elements to change the rate of impact between the peening elements and the hollow part.
Variables other than the natural frequency of vibration and proportion and material of the peening elements may also affect the impact rate of the peening elements and the hollow part. Such other variables may include the cavity height of the hollow part and the acceleration and velocity of the hollow part. What is needed is a method for establishing a relationship between these multiple variables in order to identify the optimum frequency at which to vibrate a hollow part.